In order to improve performances of automobile tires, conveyor belts and the like, steel cords are generally used as a reinforcing member.
Such steel cords are subjected to a brass plating in order to increase their adhesion to rubber (hereinafter "adhesive force") which thereby enhances their reinforcing effect.
On the other hand, a cobalt salt of an organic acid may be compounded as an adhesion promoter in a rubber composition in order to increase the adhesive force between rubber and steel cord.
However, the use of cobalt is not desirable for safety concerns and due to its high cost. And also, in case of using a cobalt salt of an organic acid, the adhesion property just after vulcanization is excellent, but the adhesion property after thermal aging is poor.
When the other metal salts of organic acids are compounded instead of the cobalt salt of the organic acid, the adhesive force just after vulcanization is poor. Therefore, metal salts of organic acids have not yet been put into practical use as a replacement for cobalt salts.